That would be inside diameter. The lip on my cat has torn and no long seals. I would like to insert a peice of pipe between the cat and the header pipe. What are the I.D.'s of them? Thanks.

But what if the I.D.'s of the header pipe and the cat neck aren't exactly the same?

Here's an idea to consider - If the neck on the front of the cat is OK (other than a bad flange) take the cat down town to a muffler shop and have them mushroom a flange on one end of a short piece of new pipe that either slips nicely into the inside (I.D.) of the cat neck, or onto the outside (O.D.) of the neck. If they charge more than $10 to do this, they're rip-offs. Then trim the old flange off the neck, noting the original length. Fit the newly created neck to the cat neck and clamp. It might be better to go for a piece of pipe that slides on the OUTSIDE of the old neck, as long as the three-bolt bracket thingy still fits OK. It seems that most of the non-OEM doughnut seals you buy are boarderline too large for the flange, so that would help that situation. Plus you wouldn't be reducing the I.D., not that it would really affect engine performance anyway.

You can cut a couple slots in the new pipe at the opposite end of the flange as pipes normally have to allow the pipe to "collapse" inside the clamp. That's another good reason to go for a piece of pipe that slips O.D. rather than I.D. - otherwise you then have to slot the cat neck which would be a bit more sacreligeous.

Whatever you do, discourage the muffler shop from welding a new flange on to the cat neck. I believe the OEM cats are stainless steel.